Rivers of rock and gas froze ancient animals in time

SKELETAL STORY An antelope-sized Psittacosaurus dinosaur (left) and two crow-sized Confuciusornis bird fossils (center and right) show postures common among entombed animals, suggesting the creatures perished in a sudden flow of volcanic rock and gas.

The Yixian and Jiufotang fossil beds of northeastern China have yielded such treasures as the first beaked bird skeleton and impressions of soft tissues, hair and feathers from prehistoric mammals and reptiles. Now, evidence suggests that pyroclastic flows — high-speed rivers of rock and gas —buried these ancient creatures and left them well preserved for 120 million years.

How the early Cretaceous creatures settled on ancient lake bottoms and became mixed with volcanic debris has been a mystery. Scientists had speculated that the animals’ corpses were flushed into lakes by rivers and then covered with ash, or that a massive volcanic eruption suffocated birds mid-flight.

But tiny mineral grains mixed among the fossils suggest pyroclastic flows, a team led by Baoyu Jiang of Nanjing University in China writes February 4 in Nature Communications. In addition, many skeletons assume a bent-limb posture typical for creatures entombed by sudden volcanic outpourings. The bones also show cracks that resemble those on human skeletons at Pompeii, which were buried after the eruption of Mount Vesuvius.